Holger L. Meyerheim
MPI für Mikrostrukturphysik, Halle, Germany
CNR – Area della Ricerca di Tor Vergata
Via del Fosso del Cavaliere 100
Transition metal oxides provide a wide variety of functionalities, which make them promising candidates for future oxide electronics . In this context, the controlled and reversible metallization is a key process. In recent time two mechanisms have been intensely investigated, namely (i) the formation of a two-dimensional electron gas (2DEG), which forms at the interface between a non-polar (e.g. SrTiO3) and a polar oxide (e.g. LaAlO3)  and (ii) the insulator to metal transition induced by the (reversible) formation of oxygen vacancies in VO2 using ionic liquid (IL) gating . The understanding of the mechanisms, which are responsible for the desired functionalities requires a precise knowledge of the geometric film and interface structure at the atomic scale.
In this Keynote Lecture, I will first discuss the complex interplay between cation intermixing at the SrTiO3/LaFeO3 interface and the IL gating induced controlled and reversible presence of oxygen vacancies, which is decisive for the formation of the 2DEG if the LaFeO3 film thickness exceeds three layers . Depending on the SrTiO3(001) surface preparation (annealed or non-annealed) a conducting or insulating interface is observed which is traced to a different cation intermixing at the interface .
In the second part of the talk, the metal to insulator transition in thin (≈10 nm) thick VO2 films on bulk TiO2 is discussed. IL gating induces a transition from the insulating (monoclinic) to the metallic (tetragonal, rutile-type) crystal structure along with a simultaneous unusual expansion of the c-lattice parameter by about 3% . An unprecedentedly precise x-ray analysis of the atomic film structure provides direct evidence for defects at the oxygen lattice site while simultaneously interstitial oxygen atoms could be located near the titanium site.
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 P. Xu, W. Han, P.M. Rice, J. Jeong, M.C. Samant, K. Mohseni, H.L. Meyerheim, S. Ostanin, I. V. Maznichenko, I. Mertig, E.K.U. Gross, A. Ernst, and S.S.P. Parkin, Adv. Mater., 1604447 (2017)
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